.outer {
    position: relative;
    height: 400px;
    width: 400px;
    background-color: red;
    .inner {
        position: absolute;
        height: 200px;
        width: 200px;
        background-color: pink;
        margin:auto;
        top: 0;
        bottom: 0;
        left: 0;
        right: 0;
        // &代表上一级.inner
        &:hover{
            background-color: deeppink;
        }
    }
}

